I can just about remember my first Adwords campaign. It was for my own business and I was spending my own money.

In my opinion, this is the best way to learn the platform - there is no better motivation to learn the subtleties (and not so subtle pitfalls) than knowing that every wasted click is money straight out of your own pocket.

In fact, for everyone wondering how best to learn AdWords, I would always suggest they look to get some skin in the game.

Looking back, I now realise how green I was and how much money I probably wasted, but that is not the theme of today's article.

Instead, today, I am considering the way in which the KPI's change over time as your Adwords knowledge develops.

As a complete novice, I was completely obsessed with the cost per click. This seemed to be the one number that determined exactly how much I was going to pay and how many visits I was likely to drive to my website.

That quickly morphed into a focus on the click through rate, since a better click through rate seemed to indicate a better resonance between my target market and the keywords and ads I was promoting.

As I learned more I discovered the concept of Quality Score. QS is, perhaps, the biggest red herring in the Adwords ocean.

Is QS important? Well, yes... but then again not in an of itself. QS is a reflection of other things - it doesn't exist in the same way that CTR and CPC exist.

Perhaps this is why Google hide QS values in the attributed section of the custom columns configuration. Something which Google tell us all the time is the cornerstone of a successful account, is, for some inexplicable reason, hidden from the default view.

Once discovered, and precisely because it is "hidden" QS is considered by many to be the Holy Grail of AdWords management. Account managers discard keywords with low QS scores. And yet, QS doesn't follow the same rules as other performance indicators. Because it isn't a performance indicator in the same way as others, it's nature is ephemeral and abstract in a way that is strangely "unscientific" coming from Google.

There is, of course, an algorithm that calculates QS - a secret akin to the recipe for Coca-Cola - but the result is fluid, and what is reported in the AdWords interface is an indication at best as QS is calculated on the fly for each search result.

We know some of the ingredients - historical CTR, expected CTR, landing page quality, ad copy, etc... and we can see that the higher the QS the better a keyword tends to perform in terms of ad position, and CPC.

But this is where the QS facade collapses. As mentioned above, QS is merely a reflection of other values.

A more mature account manager will perhaps pay lip service to QS - and it will be something that will flag a particular keyword (if a QS is 3 or 4, then we should take a look at the account to try and figure out why it is so low). But we do not fix QS. We fix other things which contribute to the QS of a keyword.

By far the most useful KPI for any account is the cost per conversion. Every campaign and website is designed to achieve an objective. A sign up, a download, a phone call, a contact form... something. And these things have a value to the company. Once we can determine how much each conversion is worth, we can then decide how much we are prepared to pay for them.

As we work towards the cost per conversion goal so we group our keywords more tightly around themes, we write better ad copy, we develop better landing pages and our conversion rate rises and our cost per conversion drops.

And, as a by-product, our QS improves. Or not. But at this point it actually matters little because we are focussing on the business objectives that truly matter.